Effects of Cr Content on Microstructure and Mechanical Properties of AlCoCrxFeNi High-Entropy Alloy

In this study, we investigated the effects of Cr content on the crystal structure, microstructure, and mechanical properties of four AlCoCrxFeNi (x� 0.3, 0.5, 0.7, and 1.0, in molar ratio) high-entropy alloys. AlCoCr0.3FeNi alloy contains duplex phases, which are ordered BCC phase and FCC phase. As the Cr content increases to x� 1.0, the FCC phase disappears and the microstructure exhibits a spinodal structure formed by a BCC phase and an ordered BCC phase. 2is result indicates that Cr is a BCC former in AlCoCrxFeNi alloys. With increasing Cr content, the alloy hardness increases from HV415 to HV498. AlCoCr0.3FeNi, AlCoCr0.5FeNi, and AlCoCr0.7FeNi exhibit a high compressive fracture strain of about 0.24 because of the formation of the FCC phase in the BCC matrix. Moreover, the highest yield stress of 1394MPa and compressive strength of 1841MPa presented by AlCoCrFeNi alloy are due to the existence of a nano-net-like spinodal structure.


Introduction
For a long time, the designs of traditional alloys, such as Al-, Mg-, Fe-, Ni-, and Co-based alloys, were mainly based on the use of one element as a principal component and the addition of minor elements for improving properties.It is because conventional metallurgical theories suggest that the use of multi-principal-element alloys may result in the formation of numerous complex structures and intermetallic compounds, which make them difficult to analyze microstructure and cause deterioration of mechanical properties.ese problems make traditional alloys difficult to break the existing applications [1].In 2004, Yeh et al. proposed an innovative concept of alloy design, the "multi-principal-element high-entropy alloys" [2], which aided development of more alloy systems with special properties and thus might enable the exploration of newgeneration alloys.High-entropy alloys are the alloys that have at least five principal elements, with the concentration of each element being between 5 and 35 at.%. Minor elements can also be added to the alloys to improve their properties.
Early studies [2][3][4][5][6][7][8][9] mostly focused on Al x CoCrCuFeNi high-entropy alloys which exhibited face-centered cubic (FCC) and/or body-centered cubic (BCC) crystal structure.At low Al content (x � 0-0.5), dendrite comprises a multiprincipal-element FCC phase, while interdendrite comprises a Cu-rich FCC phase.At higher Al content (x > 0.8), duplex FCC and BCC phases form in the interdendrite, and the spinodal phase exists in the dendrites.With increasing Al content of the alloy, the volume fraction of interdendrite gradually decreases and a net-like structure formed by spinodal decomposition is observed in dendrites, especially at x ≥ 1.0.At x ≥ 2.8, the alloy structure exhibits a single ordered BCC phase.With increasing Al content of the alloy, the hardness of the alloy increases because of the decrease in the amount of soft FCC phase and the increase in the amount of hard BCC phase.ese strengthening effects are mainly attributed to both solid solution strengthening by Al addition and nanoprecipitation hardening.Previous studies [3,10] have evidently shown that Cu has a larger positive mixing enthalpy with other elements.Hence, it easily segregates in interdendritic regions, resulting in deterioration of mechanical properties.erefore, elemental Cu has been less added to high-entropy alloys in recent years.Studies on Cu free Al x CoCrFeNi high-entropy alloys [11][12][13][14] have shown that, at an Al content x of 0-0.375, the alloys exhibit a single FCC phase, and at x ranges of 0.5-0.75 and 0.875-2, the FCC + BCC duplex phases and the BCC phase, respectively, are observed.In addition, the lattice parameter and hardness of alloys increase with increasing Al content, implying that the main two strengthening factors are the increase in (1) the hard BCC phase volume fraction and (2) lattice distortion due to the addition of Al, which has larger atomic radius than other constituent elements.
Recently, signi cant attention has been paid to highentropy alloys because of their high mixing entropy and sluggish di usion e ects, which facilitate the formation of a single solid-solution structure of the alloy instead of a complex microstructure or an intermetallic compound.As compared with conventional alloys, high-entropy alloys with an appropriate design exhibit superior performances.Studies have reported several attractive properties of high-entropy alloys, such as high strength, high malleability, good thermal stability, and high corrosion resistance [2,5,[15][16][17][18].e e ects of the alloying elements Al and Ti, which are added in AlCoCrFeNi alloy, have been widely investigated because of their excellent room-temperature mechanical properties, which are even superior to most of the high-strength bulk amorphous alloys [11,18].However, studies on the e ect of Cr content on the AlCoCrFeNi alloy have been less reported.In the present study, we investigated such e ects on the crystal structure, microstructure, and mechanical properties of four high-entropy alloys with a nominal composition of AlCoCr x FeNi (x 0.3, 0.5, 0.7, and 1.0, in molar ratio).
e dimensions of the resulting solidi ed ingot were approximately 65 mm × 40 mm × 12 mm.After the ingots were sliced, they were polished and then etched with aqua regia (HNO 3 + 3HCl) for microstructural observation by scanning electron microscopy (SEM; Hitachi S4800) equipped with backscattered electron (BSE) imaging.e chemical composition of the ingot was analyzed by energy-dispersive spectrometry (EDS).Crystal structures were characterized by X-ray di ractometer (XRD, Bruker D8 SSS) using Cu Kα radiation.Scanning was done within a 2θ range of 20 °-100 °at a rate of 2 °/min.A compression test using a ϕ5 mm × 10 mm specimen under a strain rate of 1.6 × 10 −4 s −1 was conducted on a Chun-Yen CY-6040A4 universal tester.Macrohardness and microhardness were measured by using a Vickers hardness tester (Future-Tech FR-300e) under loads of 1 kg and 5 g, respectively, for duration of 15 s.

Results and Discussion
3.1.Crystal Structure.Figure 1 shows XRD patterns of the AlCoCr x FeNi alloys.Cr-0.3 and Cr-0.5 alloys exhibit duplex phases of a major BCC phase and a minor FCC phase.Cr-0.7 and Cr-1.0 alloys exhibit a single BCC phase.Furthermore, the di raction peak for ordered BCC (100) could be detected in all alloys.Lattice parameters of the BCC and FCC phases are estimated to be 2.867 and 3.586 Å, respectively, by extrapolating the calculations of all di raction peaks of each phase [19].With increasing Cr content, it is apparent that the Cr acts as a BCC former in AlCoCrFeNi alloy due to the transition of crystal structure from BCC + FCC duplex phases to a single BCC phase. 1 display SEM-BSE images and EDS results for Cr-0.3 to Cr-1.0 alloys.For Cr-0.3 alloy, it exhibits a polycrystalline structure.Some white lath-like phases form along the grain boundaries which are similar to Widmanstätten austenite found in stainless steels [20,21].According to the EDS results in Table 1 and the XRD patterns in Figure 1, the constituent of the matrix (sign A) is similar in composition to the alloy, and the white phase across the grain boundary (sign B) is an Al-lean phase.According to XRD investigation shown in Figure 1, it is suggested A is an ordered BCC phase and B is an FCC phase.Figures 2(c) and 2(d) show the microstructure of the Cr-0.5 alloy, which is similar to that of Cr-0.3 alloy, except for the presence of more abundant white FCC phases.As shown in Figures 2(e) and 2(f), Cr-0.7 alloy also exhibits a polycrystalline structure, and the white FCC phases in amounts are less than those observed in Cr-0.5 and Cr-0.3 alloys around the grain boundary.e FCC di raction peak for Cr-0.7 alloy could not be observed because the phase presents in low amounts (Figure 1).Cr-1.0 alloy still shows a polycrystalline structure; however, the white FCC phase vanishes (Figure 2(g)).High-resolution image in Figure 2(h) shows a nano-net-like structure, which exhibits the characteristic morphology of spinodal structure reported in Al-Co-Cr-Fe-Ni-(Cu) high-entropy alloy systems [5,12,13,22,23].According to the EDS results in Table 1, the dark region (sign A) is a (Cr,Fe)-rich phase, whereas the white region (sign B) is an (Al,Ni)-rich phase.A previous study [12] revealed this Figure 3 shows the variations in volume fraction of FCC phase for the AlCoCr x FeNi alloys measured by MDS-Pro image analysis software.e volume fraction of FCC phase increases slightly, proceeding from Cr-0.3 to Cr-0.5 alloys; this trend implies that Cr plays a role of FCC former.Nevertheless, the volume fraction of FCC phase drastically decreases from Cr-0.5 to Cr-1.0 alloys, indicating that Cr is a BCC former.e first variation in the increase of FCC phase from Cr-0.3 to Cr-0.5 alloys is contrary to previous observations in which Cr is a BCC former in high-entropy alloys [14].In the investigation of FCC and BCC equivalents of elements in high-entropy alloys, Ke et al. [24] reported that 2.23 Cr BCC is equivalent to Al BCC for the stabilization of the BCC phase.Similar result was popularly applied in stainless steel proposed by Hull [25].Table 1 lists the compositions of all alloys.As shown, the  for the alloys Cr-0.5 to Cr-0.7, which is much greater than 2.23, implying an increase in BCC equivalent.Hence, the volume fraction of the FCC phase greatly decreases.Finally, the FCC phase vanishes in the Cr-1.0 alloy, as the Cr inc /Al dec ratio for the alloys Cr-0.7 to Cr-1.0 is 5.15.ese results con rm that Cr has weaker ability to form the BCC phase as compared with Al in high-entropy alloys.

Hardness.
Figure 4 shows the variations in alloy hardness, ordered BCC phase hardness, and FCC phase hardness for the AlCoCr x FeNi alloys.e alloy hardness linearly increases with increasing Cr content.Hardness values for the alloys Cr-0.3,Cr-0.5, Cr-0.7, and Cr-1.0 are HV415, HV431, HV448, and HV498, respectively.ose for the ordered BCC phase of Cr-0.3, Cr-0.5, and Cr-0.7 alloys are HV425, HV446, and HV452, respectively.Hardness values for the FCC phase of Cr-0.3 and Cr-0.5 alloys are the same (HV252).e Cr-0.7 alloy, with hardness of HV343, contains both FCC and ordered BCC phases because the FCC phase is too small to be measured by our hardness tester.
e hardness curves suggest that the hardness of alloys Cr-0.3 to Cr-0.7 mainly comes from the ordered BCC phase.With increasing Cr content, the hardness of the ordered BCC phase increases because of (1) the solute strengthening e ect that re ects in hardness increase [3] and (2) the highest melting point of Cr in the BCC phase, which also increases the Young's modulus and the slip resistance [6].e highest hardness of Cr-1.0 alloy is attributed to the formation of a nano-net-like spinodal structure (Figure 2(h)), which is composed of BCC and ordered BCC phases.

Compressive Properties.
Figure 5 shows true stress-strain curves obtained from the compression tests on the AlCoCr x FeNi   Advances in Materials Science and Engineering alloys.Table 2 summarizes data for the yield stress (σ y ), compressive strength (σ max ), and fracture strain (ε f ).With increasing Cr content of the alloy, both σ y and σ max increase from 1109 to 1394 MPa and from 1579 to 1841 MPa, respectively.However, ε f only decreases from 0.25 to 0.19.e strengthening mechanism is mainly attributed to the hard ordered BCC phase mentioned in Section 3.3.e slightly higher ε f of alloys Cr-0.3 to Cr-0.7 as compared with that of Cr-1.0 is due to the existence of the ductile FCC phase in the former alloys.Figure 6 shows SEM images of the fracture surfaces after the compression of the four alloys.e characteristics of the river pattern and tear ridge shown in Figures 6(a) to 6(d) indicate a quasicleavage fracture in all alloys during compression, which accounts for the small difference in ductility of the four alloys.As shown in Table 2, σ max and ε f of alloys Cr-0.3 to Cr-1.0 are better than those of other high-entropy alloys and bulk amorphous alloys [18,[26][27][28][29][30].ese properties indicate the potential use of this alloy system in the development of structural materials with high strength and ductility.Advances in Materials Science and Engineering

Conclusions
AlCoCr 0.3 FeNi, AlCoCr 0.5 FeNi, and AlCoCr 0.7 FeNi alloys contain duplex phases in which major ordered BCC and minor FCC phases are observed.AlCoCrFeNi alloy exhibits a nano-net-like spinodal structure consisting of an (Al,Ni)rich ordered BCC phase and a (Cr-Fe)-rich BCC phase.Cr is a BCC former in AlCoCr x FeNi alloys.e hardness, yield stress, and compressive strength of the alloys increase with the increase in Cr content.However, the fracture strain only decreases slightly.e present four AlCoCr x FeNi alloys exhibit a combination of high strength and high ductility.

Figure 3 :Figure 4 :Figure 5 :
Figure 3: Variation in volume fraction of FCC phase for the AlCoCr x FeNi alloys.
Advances in Materials Science and Engineering increase in Cr content (Cr inc ) is 3.63 at.% and the decrease in Al content (Al dec ) is 1.65 at.% for alloys Cr-0.3 to Cr-0.5.e Cr inc /Al dec ratio is 2.2, which is less than 2.23 and thus indicates that the BCC equivalent decreases from Cr-0.3 to Cr-0.5 alloys.us, the volume fraction of the FCC phase slightly increases at this time.However, the Cr inc /Al dec ratio is11.42